A display apparatus that is capable of producing at least two different images simultaneously is referred to herein as a directional display apparatus. A directional display produces at least two different images, each of which is viewed from a different viewing position. In one type of directional display device, the two images are intended to be viewed as distinct separate images. Such a display may also be referred to as “a multi-viewer display,” “multi-view display” or “multi-user display” which may be configured so that different observers see different images. This allows for multiple simultaneous uses of the display. A multi-view display may also be configured for use by a single observer.
A directional display may also be configured to produce at least two separate images that are intended to be fused into a single image by the observer. Normal human vision is stereoscopic such that each eye sees a slightly different image of the world. The human brain fuses the two images (referred to as the stereo pair) to give the sensation of depth in images observed in the real world. In a three dimensional display apparatus, a separate image is provided to each eye, and the brain of the observer fuses the stereo pair of images to give the appearance of depth in the fused image.
A three dimensional display apparatus is typically classified as being either stereoscopic or autostereoscopic. In a 3D stereoscopic display apparatus, some kind of viewing aid is worn by the user to substantially separate the views sent to the left and right eyes. For example, the viewing aid may be color filters in which the images are color coded (e.g. red and green), polarizing glasses in which the images are encoded in orthogonal polarization states, or shutter glasses in which the views are encoded as a temporal sequence of images in synchronization with the opening of the shutters of the glasses. In contrast, a 3D autostereoscopic display apparatus operates without the need for the observer to wear a viewing aid. In autostereoscopic displays, each of the views can be seen from a limited region in space.
Overview of Directional Display Devices
U.S. Pat. No. 7,058,252, entitled “Optical Switching Apparatus” and issued to Woodgate et al., provides a comprehensive discussion of the technical features and issues related to directional displays, and in particular autostereoscopic 3D displays. The subject matter at columns 1 through 8, as well as the figures referred to therein, of U.S. Pat. No. 7,058,252 are incorporated herein by reference for all that they teach. In general, an autostereoscopic system comprises a display panel and an optical steering element or mechanism for directing the light from at least two separate images. The optical steering mechanism may also be referred to as an optical director, parallax optic, or parallax barrier. The optical steering mechanism sends the light from a left image to a limited region in front of the display panel, referred to as a first viewing window. When the observer places their left eye at the position of the first viewing window, then the observer sees the appropriate image across the whole of the display panel. Similarly, the optical steering mechanism sends the light intended for the right image to a separate second viewing window. When the observer places their right eye in the second viewing window, the right eye image will be seen across the whole of the display. Generally, the light from either image may be considered to have been optically steered (i.e. directed) into a respective directional distribution. The viewing window plane of the display represents the distance from the display at which the lateral viewing freedom is greatest.
FIG. 1 herein illustrates an exemplary flat panel autostereoscopic display 10 as shown in FIG. 5 of U.S. Pat. No. 7,058,252. Display 10 comprises a backlight, an array of electronically adjustable pixels (known as a spatial light modulator, SLM) arranged in columns and rows and a parallax barrier attached to the front of the display which acts as the optical steering mechanism. The term “spatial light modulator” includes both light valve devices such as liquid crystal displays and emissive devices such as electroluminescent displays and LED displays. A backlight 60 produces a light output 62 which is incident on an LCD input polarizer 64. The light is transmitted through a TFT LCD substrate 66 and is incident on a repeating array of pixels arranged in columns and rows in an LCD pixel plane 67. The red pixels 68,71,74, green pixels 69,72,75 and blue pixels 70,73 each comprise an individually controllable liquid crystal layer and are separated by regions of an opaque mask called a black mask 76. Each pixel comprises a transmissive region, or pixel aperture 78. Light passing through the pixel is modulated in phase by the liquid crystal material in the LCD pixel plane 74 and in color by a color filter positioned on an LCD color filter substrate 80.
The light then passes through an output polarizer 82 after which is placed a parallax barrier 84 and a parallax barrier substrate 86. In FIG. 1, the parallax barrier 84 comprises an array of vertically extended transmissive regions separated by vertically extended opaque regions and serves to direct light from alternate pixel columns 69,71,73,75 to the right eye as shown by the ray 88 for light from pixel 69 and from the intermediate columns 68,70,72,74 to the left eye as shown by the ray 90 (this overall light direction pattern forming another example of a directional distribution of light). The observer sees the light from the underlying pixel illuminating the aperture of the barrier, 92. Other types of optical directors or parallax optics may be used in 3D displays, such as a lenticular screen and birefringent lenses.
With continued reference to FIG. 1, the repeating array of pixels arranged in columns and rows in LCD pixel plane 67 are separated by gaps, (generally defined by the black mask in a liquid crystal display, LCD) with the parallax barrier being an array of vertically extended slits of pitch close to twice the pitch of the pixel columns. The parallax barrier limits the range of angles from which light from each pixel column can be seen, thus creating the viewing windows at a region in front of the display.
In order to steer the light from each pixel to the viewing window, the pitch of the parallax barrier is slightly smaller than twice the pitch of the pixel array. This condition is known as “viewpoint correction”. In the type of display illustrated in FIG. 1, the resolution of each of the stereo pair images is half the horizontal resolution of the base LCD, and two views are created. Thus, the light from the odd columns of pixels 68,70,72,74 can be seen from the left viewing window, and the light from the even columns of pixels 69,71,73,75 can be seen from the right viewing window. If the left eye image data is placed on the odd columns of the display and the right eye image data on the even columns then the observer in the correct “orthoscopic” position should fuse the two images to see an autostereoscopic 3D image across the whole of the display.
U.S. Pat. No. 7,154,653 entitled “Parallax Barrier and Multiple View Display” and issued to Kean et al., discloses various embodiments of parallax barriers for use in both multi-user and 3D displays. The background discussion of U.S. Pat. No. 7,154,653 and the figures referenced therein from columns 1 through column 5, which are hereby incorporated herein by reference, discuss the characteristics of parallax optics that may be varied or modified in order to control the size of, and the angle between, viewing windows, or viewing regions, to which the multiple images (e.g., left eye and right eye) produced by the display are directed. The function of the parallax optic is to restrict the light transmitted through the pixels to certain output angles, thereby defining the angle of view of the pixels behind a specific part of the parallax optic structure (e.g., a slit or lenslet or lenticule.) For flat panel autostereoscopic displays, the formation of the viewing regions is typically due to a combination of the pixel structure of the display unit and the light-directing optical element, or parallax optic.
U.S. Pat. No. 7,154,653 discloses the display 30 shown in FIG. 2A herein. Display 30 is a two view directional display which may be used as an autostercoscopic 3D display or as a display for providing two unrelated views to one or more observers. The display comprises a spatial light modulator in the form of a liquid crystal display (LCD) 20. LCD 20 is pixellated, which is defined herein to indicate a display that substantially comprises a subpixel repeating group of at least two primary color subpixels. LCD 20 operates in transmissive mode so as to modulate light passing through the subpixels from a backlight (not shown). However, U.S. Pat. No. 7,154,653 notes that other types of display may be used to modulate light in transmissive or reflective modes or to generate light within the display device itself (in the case of a front parallax barrier arrangement). Display 30 also comprises a parallax barrier 21 disposed in front of LCD 20, i.e. between the LCD 20 and the viewer or viewers. Barrier 21, illustrated in more detail in FIG. 2B herein, provides regions 22 and 23 which are substantially opaque to light from the LCD 20 and slits therebetween which are substantially transmissive to light from the LCD 20. The regions 22 and 23 have a finite width and all of the slits have the same maximum light transmission. The columns of subpixels of LCD 20 are formed with a substantially uniform pitch p in a direction perpendicular to the longitudinal axes of the columns, the direction of which is generally horizontal during normal use of the display. The slits of barrier 21 are arranged non-periodically, extend parallel to the longitudinal axis of the subpixel columns, and are arranged in evenly spaced groups of slits with the slits in each group being evenly spaced. FIG. 2A also shows details of an illustrated embodiment of parallax barrier 21 with respect to the size and arrangement of the slits.
With continued reference to FIG. 2A, display 30 is driven by display driver 25 such that image data for the two views which are to be displayed are interlaced as vertical stripes. The display driver 25 may be arranged to receive images for display and to interlace the data so as to ensure that the individual pixel columns display the correct vertical slices of the images. The display driver 25 may form part of the display or may be embodied partly or wholly within other apparatus, such as a computer, microprocessor or the like. The images may be captured “real” images or may be computer-generated. The images may form a stereoscopic pair for autostereoscopic 3D use of the display or may be stereoscopically unrelated images. The slits of barrier 21 are aligned with or adjacent to the middle lines of the columns of pixels. Display driver 25 supplies vertical image slices to a group of four columns of pixels nearest each group of slits. The slits of the barrier 21 cooperate with the pixellation of the LCD 20 so as to define or create five viewing regions. In each of the viewing regions, each group of slits restricts visibility of the columns of pixels such that two adjacent pixel columns only are visible to a viewer viewing the display from the viewing region.
With reference to FIG. 2C, display driver 25 supplies the pixel image data to the LCD 20 such that first and second image slices are provided from one of the images and third and fourth image slices are provided from the other of the images. Thus, the first and second images forming the first and second views are visible in the viewing regions D and B, respectively. When providing autostereoscopic viewing, provided the left and right eyes of the observer are in the viewing regions B and D, respectively, a stereoscopic pair of images can be viewed correctly to provide the 3D effect. Conversely, a viewer whose eyes are in the viewing region D can see one of the images but not the other whereas a viewer whose eyes are in the region B can see the other image but not the first image. The viewing regions to either side of the regions B and D which are actually used contain 50% of each image, reducing the contribution to crosstalk from neighboring viewing regions. Display 30 makes use of 50% of the available light and each image is displayed by 50% of the subpixels so that the horizontal resolution is 50% of the LCD resolution.
U.S. Pat. No. 7,058,252 referenced above also describes a type of display that is capable of operating in both 3D and two-dimensional (2D) modes. This type of display is referred to therein as a “2D 3D switchable display” and U.S. Pat. No. 7,058,252 discusses several examples of such displays, one of which is illustrated in FIGS. 3A and 3B herein. FIG. 3A shows a display comprising a backlight 60, producing light output 62 which is incident on an LCD input polariser 64, an LCD TFT substrate 66, an LCD pixel plane 67 comprising an array of pixels arranged in columns and rows followed by an LCD counter substrate 80, an array of birefringent lenses 138, followed by an isotropic lens microstructure 134 followed by a lens substrate 132. The previous items can be grouped as a directional display device 236. Following the directional display device 236, a polarization modifying device 146 is positioned. One illustrative configuration of the operation of the display in the 2D mode is also shown along the propagation direction 238. The polarization modifying device 146 transmits horizontal linearly polarized light and extinguishes vertically polarized light. The LCD input polarization 240 is at 90-degrees and is rotated by the ON state of the liquid crystal material in the pixel apertures 78 to a horizontal polarization (0-degrees angle) 242 by the twisted nematic layer, thereby providing a normally white (NW) mode. In the NW mode ON state, no voltage is applied to the liquid crystal layer. Voltage is applied to switch the output to an OFF state, or intermediate levels. The birefringent microlenses 138 are index matched in this polarization and so do not impart any directionality to the illumination. The output of the polarization modifying device 146 is horizontal linear polarization 244.
FIG. 3B shows the configuration for the 3D operation of the display shown in FIG. 3A, along the propagation direction 238. In this case, the polarization modifying device 146 is arranged to transmit vertically linearly polarized light and extinguish horizontally polarized light. The LCD input polarization 240 is at 90-degrees and is not rotated by the ON state of the liquid crystal material to a horizontal polarization (0-degrees angle) 242 by the twisted nematic layer, thereby providing a normally black (NB) mode. In the NB mode ON state, voltage is applied to the liquid crystal layer. Reducing voltage is applied to switch the output to an OFF state, or intermediate levels. The polarization state 246 incident on the birefringent microlenses 138 is given directionality by the birefringent lens 138. In this case, the polarization modifying device 146 is configured to transmit vertical linear polarization state 248, such that the 3D mode illumination structure is transmitted.
Additional information about three-dimensional displays may be found in chapter 2.6 in the Handbook of Optoelectronics, Dakin and Brown, eds., Vol. II, entitled “Three-dimensional display systems,” published by CRC Press (2006), which chapter is hereby incorporated by reference herein.
U.S. Pat. No. 7,058,252 referenced above also describes a multi-user display embodiment, shown herein in FIG. 4. FIG. 4 shows in plan view a birefringent microlens display 406 producing viewing windows 408, 410, 412 and 414. The window size is arranged to be greater than the interocular separation of the viewers. Display 406 is suitable for use, for example, on the dashboard of an automobile. The driver places his right eye 416 in window 408, and also his left eye 418 in the same window 408. Similarly the passenger places his left eye 422 and right eye 420 in a single window 414. For a two view display, windows 408 and 412 contain the same information and windows 410 and 414 contain the same information. For aberrational design purposes, it may be convenient to have the windows 410 and 412 between the passenger and driver of the display. If a first image 426 and a second image 428 are input, then an image signal interlacer 424 will put image 426 on the even columns of the display for example, and image 428 on the odd columns of the display for example. The optical elements of the display will direct image 426 to the driver in window 408 and image 428 to the passenger in window 414. U.S. Pat. No. 7,058,252 notes that the display operates in the same manner as the 2D 3D switchable displays described therein, except that viewing windows 408, 410, 412 and 414 are substantially larger than the viewing windows produced by the 2D 3D switchable displays to allow for different viewers to be positioned in different windows. U.S. Pat. No. 7,058,252 further notes that such multi-viewer displays may have two modes of operation: in one mode of operation all viewers can see the same image and in a second mode of operation different viewers can see different images to allow multiple simultaneous uses of the same display.
U.S. Pat. No. 6,424,323, entitled “Electronic Device Having a Display” and issued to Bell et al., also discloses an electronic device having a display and an image deflection system overlying the display, wherein the display is controlled to provide at least two independent display images which, when displayed through the image deflection system, are individually visible from different viewing positions relative to the display. One embodiment of an image deflection system disclosed is a lenticular screen comprising a plurality of lenticles (also referred to as lenticules.) The lenticles extend across the display so that different images are visible as a function of the angle of inclination of the viewer with respect to the screen. In this way, a single user may view the different images by tilting the device about a horizontal axis.
Viewing Window Performance Issues
The term “cross talk” refers to light leakage between the two views such that some of the left eye image will be seen by the right eye and vice versa. Cross talk generates visual strain when viewing 3D displays, and control of cross talk is an important factor in 3D display development. For flat panel autostereoscopic displays (in particular those based on LCD technology), the limitation of window performance is generally determined by the shape and aperture ratio of the pixel and the quality of the optical element. U.S. Pat. No. 7,058,252 referenced above notes that the angles of the output cone of light emitted from the display are determined by the width and shape of the pixel aperture and the alignment and aberrations of the parallax optic. The disclosure in U.S. Pat. No. 7,154,653 further notes that attempts to reduce cross talk (i.e., light leakage between images) by reducing the width of the slits in the parallax barrier may result in uneven color balance as more of one color subpixel becomes visible to the observer, or the color balance may change with the angle of view.
The disclosure in U.S. Pat. No. 7,154,653 further notes that, in order to increase the lateral viewing freedom of the display, more than two pixel columns can be placed under each slit of the parallax barrier. For example, four columns will create four windows in which the view is changed for each window. Such a display will give a “look-around” appearance as the observer moves. The longitudinal freedom is also increased by such a method. However, in this case, the resolution of the display is limited to one quarter of the resolution of the base panel. Moreover, parallax barriers rely on blocking the light from regions of the display and therefore reduce the brightness and device efficiency, generally to approximately 20-40% of the original display brightness.
U.S. Pat. No. 7,154,653 discloses that LCD 20 shown in FIG. 2A herein is of a “conventional” type of display in which “white” pixels are divided into repeating groups of color sub-pixels. In particular, the pixel columns of each group of three columns are provided with red, green and blue filter strips so that all of color sub-pixels in each column display the same color and adjacent pairs of columns display different colors with the pattern red (R), green (G) and blue (B) repeating across the display. U.S. 7,154,653 notes that, although the correct color balance is obtained for the right and left views with such an arrangement, there is a substantial non-uniformity in the spacing of single colors for each view. Such uneven spacing can be very visible in low resolution displays, and therefore detracts from image quality. Also, for each view, the ordering of the color sub-pixels does not follow the same repeating pattern of the three color sub-pixels that comprise LCD 20; this is referred to as “crossing over” in the ordering of the components of each white pixel, and such crossing over can lead to further undesirable image artifacts. U.S. Pat. No. 7,154,653 further discloses examples of alternative subpixel arrangements or layouts other than the standard repeating RGB sub-pixel arrangement. One such arrangement provides for no crossing over in the ordering of the sub-pixel components of a pixel that produces a white color and reduces the spacing of individual color sub-pixels in each view, with a goal of improving the image quality.
U.S. Pat. No. 6,023,315 entitled “Spatial light modulator and directional display” and issued to Harrold et al., discloses a liquid crystal spatial light modulator comprising columns and rows of picture elements, arranged as groups of columns, for instance under respective parallax generating elements in an autostereoscopic 3D display. The picture elements are arranged as sets to form color picture elements such that the picture elements of each set are disposed at the apices of a polygon, such as a triangle, and are disposed in corresponding columns of the groups of columns. U.S. Pat. No. 6,023,315 comments on the deficiencies of using a spatial light modulator having the conventional RGB vertical or horizontal stripe subpixel arrangement, or the known RGGB quad subpixel arrangement, to produce stereoscopic images for a 3D display, citing problems with color integration. In order to alleviate these problems, U.S. Pat. No. 6,023,315 discloses various embodiments of sub-pixel arrangements and groupings of sub-pixels, called “tessellations,” that are designed so that color integration occurs over a substantially larger range of viewing distances. One set of several such arrangements makes use of red, green, blue and white sub-pixels.